To produce solar cells, semiconductor layers having differing dopings are required, which include at least one Schottky or p-n junction. As is known from the prior art (WO 2011/061106 A2), a pin junction can be generated by depositing an amorphous layer sequence, containing higher silanes, among other things, on a suitable substrate using liquid phase deposition, the junction acting as a solar cell with suitable front and rear contacts. In the production of thin-film solar cells based on p-i-n junctions, the individual doping layers are doped as an n-silicon layer with a suitable phosphorus compound, for example, such as white phosphorus, and as a p-silicon layer with a suitable boron compound, for example, such as decaboron, wherein the suitable phosphorus or boron compounds can be added to the liquid phases. These may be liquid themselves or dissolve in the liquid phase. No additional dopants are added when an intrinsic (i-), which is to say undoped, silicon layer is being produced.
In general, compositions comprising hydridosilanes, among other things, are used to produce silicon-containing layers based on liquid silane. Prior to being applied to the respective substrate, these compositions are initially polymerized into higher hydridosilanes.
In general, a polymerization by way of electromagnetic radiation (such as by way of ultraviolet (UV) light) or by supplying heat is referred to as a ring-opening polymerization when the reactant is composed of cyclic monomers, such as cyclopentasilane (Si5H10) or cyclohexasilane (Si6H12). These are cleaved using the action of light or heat, and reacted alone or by further bonding other hydridosilanes to form linear and/or branched polymer chains made of silicon and hydrogen. The resulting hydridosilanes are denoted by the general formula —(SiH2)n— and also referred to as polysilanes or oligosilanes. Hydridosilanes having a molar mass of less than approximately 302 g/mol (which is to say≤10 Si atoms) are classified as “lower hydridosilanes” and those having higher molar masses as “higher hydridosilanes.”Hydridosilanes present in liquid form are also referred to as liquid silane.
Silicon thin layers based on liquid silane are normally applied to a substrate by way of spin coating. It is customary, for this purpose, to dilute the liquid silane in a solvent. This solution is subsequently polymerized. In the case of cyclic reactants, the solution can be irradiated with UV light so as to enable photopolymerization of the dissolved liquid silane. The photopolymerized solution is subsequently filtered so as to remove insoluble hydridosilanes from the solution, or to control the molar mass of the photopolymerized hydridosilane. The filtered solution is subsequently applied to the substrate by way of spin coating. At the end, the layer is heated so as to evaporate the solvent and convert the silicon into an amorphous form. The latter is referred to as conversion. As an alternative to the direct wet-chemical method, it is also possible to conduct a carrier gas, such as hydrogen, through the liquid silane (bubbler system). Finally, this gas mixture is decomposed by way of plasma-enhanced chemical vapor deposition (PECVD) or hot wire chemical vapor deposition (HWCVD), and deposited on the substrate.
The resultant silicon layers can comprise a mixture of microcrystalline, polycrystalline and amorphous structures, depending on the conditions (such as process temperature, heating duration, hydrogen partial pressure) under which the conversion takes place. The crystallinity of the layer, which is typically in an amorphous state, can subsequently be increased, for example by way of laser irradiation [1] or thermal treatment (for example, temperatures higher than 600° C.).
The drawback of photochemical methods known from the prior art for producing coating solutions (precursors) for silicon layers is that they are very time- and labor-intensive. Photopolymerization is a very time-consuming method. Irradiations typically last between 10 and 120 minutes [2] [3] or up to 840 minutes [DE 10 2010 041 842 A1], and the use of UV irradiation incurs additional procurement costs, and safety measures may need to be taken.
(WO 2012/084261 A1) describes another method for depositing silicon-containing layers onto a substrate, wherein a focused beam of charged particles (ions or electron beam) is used to dissociate a polysilane-based precursor directly on a substrate. The method has disadvantages when silicon layers for optoelectronic applications are produced since it requires the use of expensive vacuum technology and a modified scanning electron microscope (SEM). As an alternative to the use of an electron beam, it is possible to use Ga+ ions instead. Here, the method has the disadvantage that the production of intrinsic silicon layers is made more difficult due to the doping property of Ga.
Hydridosilanes can also be caused to polymerize using thermal methods (DE 10 2010 041 842 A1). Here, for example, reaction mixtures made of neopentasilane Si(SiH3)4, among other things, are heated at 154° C. and thermally treated for approximately 200 to 480 minutes.
The supply of heat achieves polymerization by cleaving the Si—Si and/or Si—H bonds of the cyclic or linear or branched monomers. Higher hydridosilanes are subsequently created by chain formation. For this purpose, it is necessary to heat pure liquid silane, or a diluted liquid silane, in the form of a hydridosilane solution (=hydridosilanes diluted in a solvent) to high temperatures of up to 235° C. (WO 2011/104147 A1), so as to bring about thermal decomposition. This method has the disadvantage that it is energy-intensive and that, due to the heating process or the residual thermal energy of the solution, the polymerization cannot be immediately initiated (due to the heating process) or immediately terminated (due to the cooling process) without additional work steps.
The disadvantages of all of the above-mentioned methods for producing coating solutions for silicon layers are the time-intensive temperature and UV irradiation or filtration steps necessary for polymerization or molecular weight limitation. When the hydridosilanes are applied to the substrate by way of spin coating, it is a further disadvantage that valuable material is wasted, since the solution, as a result of the spinning, is not only distributed on the substrate surface, but due to the centrifugal force approximately 90% of if it thrown beyond the surface to be coated. Moreover, the substrate of thin layers produced by way of spin coating is inhomogeneously covered, wherein only planar, solid substrates can be used. When ultrathin layers (<2 nm) are used for coating or layers are grown in the form of monolayer coating, spin coating is not a very suitable method due to significant surface roughness and undulations of >3 nm, for example (WO 2011/104147 A1). When nanoparticles are dispersed in a solvent (such as cyclooctane, ethanol, toluene, water), in a hydridosilane solution or directly in the pure liquid hydridosilane (both in monomeric and polymeric hydridosilane), the layers produced thereafter by way of spin coating have both a suboptimal nanoparticle embeddedness and a suboptimal nanoparticle distribution. When structured surfaces are coated with nanoparticles, coverings produced by way of spin coating have a low quality, for example with an inhomogeneous nanoparticle distribution and low reproducibility. Surfaces thus structured are used in photovoltaics for light trapping purposes and as plasmonic reflection gratings, for example, so as to increase the efficiency of solar cells.
The use and the influence of ultrasonic waves and acoustic cavitation on chemical and physical processes are known from the prior art. Ultrasound is used to trigger acoustic cavitation, which is to say the formation, growth and implosion of microcavities in the liquids. The pressure prevailing in the microcavities and the effective temperature are in the range of 1000 bar and 5000° C., respectively [4]. The process temperature, which is to say the temperature that prevails in the liquid or in the reaction vessel itself, however, can remain at temperatures below 0° or at room temperature or in the range below 150° C., depending on which use is intended and which device is used in each case. Ultrasonic waves can be used for treating solutions and objects to enable purely physical effects, such as deagglomeration and dispersion of particles, formation of emulsions or ultrasonic cleaning. The chemical effects are based on phenomena triggered in microcavities and the immediate vicinity thereof, such as bond cleavage through tensile forces or high transient temperatures and radical generation. The use of ultrasound represents an alternative to traditional photochemistry, thermochemistry or catalytic chemistry. In general, chemistry supported by way of acoustic cavitation is known as “sonochemistry” and has countless applications, such as the degradation of organic polymers, polymerization of organic compounds, generation of radicals, acceleration of chemical reactions, and the like.
It is the object of the invention to provide a method for polymerizing a composition comprising hydridosilanes and subsequently using the polymers to produce silicon-containing layers, which allows a simpler and faster method for producing silicon-containing layers compared to the prior art than has been previously possible according to the prior art.
If is a further object of the Invention to make a device available that is adapted to this method, and to provide silicon layers produced by way of this method. The invention further relates to the use of these silicon-containing layers to produce semiconducting or insulating thin films with or without nanoparticles embedded therein.
Layers that are produced by way of the present invention can have higher-quality structural and electronic properties. Deposition by way of aerosol coating allows not only comparable or lower surface roughness and undulations, but also more homogeneous covering of the substrate to be achieved as compared to previously known methods.
If, according to the invention, simultaneous periodic heating of the substrate and a continuous supply of aerosol are carried out during the coating process, the amorphous layer growing in the immediate vicinity of the substrate is completely or partially protected from the influence of impurities generally present in the nitrogen atmosphere. The layers produced by the method according to the invention can be used in solar cells, photodiodes or thin-film transistors, for example.
The objects of the invention are achieved by a method having the characteristics according to the main claim, and by a device and a silicon layer and the use thereof having the features according to the additional independent claims. Advantageous embodiments of the method, the device, and the silicon layer will be apparent from the respective dependent claims.